Method and device for electrostatic charging

ABSTRACT

The invention relates to a method for electrostatic charging of the two outer sides of at least one material web with charges of opposite polarity, prior to the further processing of the at least one material web, with the web being guided through the nip of a pair of rollers that are parallel to one another and are arranged with a short space between them, characterized in that the electrostatic charging of the two outer sides of the at least one material web is performed by the rollers in the nip between them.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method as well as a device for electrostaticcharging of the two outer sides of at least one material web withcharges of opposite polarity, prior to the further processing of the atleast one material web wherein the web is guided through the nip of apair of rollers that are parallel to one another and have a smalldistance between them.

2. Description of the Prior Art

A method of this kind and a device for electrostatic charging is known(DE 31 17 419 A1). In this document, a plurality of material webs arebrought together to form a hank and only the two outer sides are chargedby means of a device designed as a corona-charging electrode forelectrostatic charging downstream from the first pair of rollers bymeans of a high voltage source of 30 kV.

The disadvantage of this known device is a charging of the two outersides of the material webs that is not highly directed in space. Thus, alarge number of charged particles migrate throughout the area wherecharging is taking place. This results in poor charging efficiency. Inaddition, there is the danger that under unfavorable environmentalconditions, the corona charging electrodes will become contaminated andconsequently fall to operate. The cleaning expense can be very high.

SUMMARY OF THE INVENTION

The invention is a method providing a more closely directed chargingwith improved efficiency.

The invention uses the unequal electrostatic charging of the two outersides of the at least one material web, performed in the nip by eachroller of the pair.

The rollers can be charged through the surface, namely by means of aso-called contact roller (DE 38 23 739 A1), or by wiper brushes known ofthemselves, or by a wiper contact.

In an advantageous improvement according to the invention, charging isperformed from the inside by the device composed of the pair of rollers,with each roller having a covering with limited electrical conductivity(so-called semiconducting) on top of a steel jacket, with the steeljacket of each roller being connected to a positive or negative highvoltage source for electrical charging of the coating with limitedconductivity. In the device according to the invention, because thevoltage is applied from the inside, retrofitting in existing systems forprocessing material webs, preferably made of plastic or paper, using ahigh voltage source of 3 to 7 kV is possible. Therefore, much lowervoltages are used relative to the prior art. In addition, charging takesplace precisely at the charged point, namely at the outside of the atleast one material web, so that efficiency is achieved that isconsiderably better by comparison with the prior art. Finally, due tothe elimination of corona charging electrodes, their cleaning cost iseliminated completely so that lower downtime expenses are achieved aswell as when the system is in operation.

Advantageously, in the invention the axis or shaft of the roller isconnected in an electrically conducting fashion with the steel jacketand is insulated electrically from the machine frame supporting theroller. For electrical charging of the coating with limitedconductivity, the high electrical voltage is applied to the axis orshaft which has a terminal for this purpose. This could be either afixed terminal on the axis or a pressure bearing located at the end ofthe shaft.

In an advantageous improvement on the invention, it is also possible tomount a primary coil permanently relative to the machine frame andconcentrically with respect to the axis or shaft of the roller at oneend, concentrically with respect to the axis or shaft of the roller andnext to the roller, and to provide a secondary coil that turns with theroller, with its one terminal on the axis or shaft and its otherterminal connected to the steel jacket through a rectifier circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Three embodiments of the invention will be described in greater detailbelow with reference to the drawing.

FIG. 1 is a first embodiment of the device according to the invention ina schematic cross section and partially broken away;

FIG. 2 is a second embodiment of the device according to the inventionin a schematic cross section and a simplified partially broken-awayview; and

FIG. 3 is a third embodiment of the device according to the invention ina partially broken-away and simplified broken-away view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Each of the two rollers 5 in FIG. 1 has a fixed axis 6 as well as asteel jacket 7, on top of which is a coating 8 which has limitedconductivity that can be charged electrically. Between axis 6 and steeljacket 7, a ball bearing 9 is provided. A nip 50 is provided between thetwo rollers at which opposed surfaces of webs of material are charged bycontact with two rollers that are mounted parallel to one another.

The axis of roller 5 is mounted in a machine stand 10, in electricalinsulation 11. Spaces 12 and 13 are sufficient to prevent discharges orvoltage sparkovers.

The end of axis 6 is connected to a generator 15, and a measuring device16 with a terminal 17, with the lead running to terminal 17 beinggrounded through a resistance 18. Machine stand 10 is likewise grounded,as shown schematically by 19.

The embodiment according to FIG. 2 differs from that in FIG. 1 in thatonly one of the two rollers is shown and axis is designed 20 which isconnected rigidly through an electrically conducting intermediate piece21 with steel jacket 7.

For this purpose, shaft 20 of machine stand 10 is connected eitherthrough an electrically insulating ball bearing 22 or separateinsulation 11. It is also possible to use a normal electricallyconducting ball bearing and to provide an electrically insulating sleeve11 between the latter and machine stand 10.

In addition, a pressure bearing 23 is provided as a terminal at end 14of shaft 20, said terminal being electrically conducting and to whoseexterior the terminal 17 of high-voltage generator 15 is connectedthrough measuring device 16.

In both embodiments according to FIGS. 1 and 2, assurance is providedthat the high voltage applied through axis 6 or shaft 20 cannot beconnected to machine stand 10 because of electrical insulation 11 andthe high voltage on coating 8 with limited conductivity of roller 5 canpass from the inside to the outside.

The additional, third embodiment shown in FIG. 3 likewise has tworollers forming a nip, however only one roller is shown for reasons ofimproved clarity, that is like the one shown in FIGS. 1 and 2. Inaddition, concentrically to shaft 6 of roller 5, and in addition to thelatter, a first receiving device 113 is provided at one end 112 as amagnetizable core with a secondary coil 114 and a second secondary coil115, each concentric to shaft 6.

An additional receiving device 116 is provided concentrically withrespect to shaft 6 as a magnetizable core for receiving a primary coil117, likewise concentric with respect to shaft 6, with electricalterminals 1 and 2 as well as a second primary coil, with electricalterminals 3 and 4 located between primary coil 117 and shaft 6.Receiving device 16 is rotatable relative to shaft 6 by means of a pinthat engages an anchor 120 that is fixed relative to the machine frame,and has on its interior a ball bearing designated as a whole by 121, sothat the device can turn but the magnetizable core is held nonrotatablyrelative to first receiving device 113 by pin 119 and anchor 120.

On the secondary side of first receiving device 113, a rectifier circuit122 and a smoothing circuit 123 are also provided, with circuit 123having its output connected to coating 8 with limited conductivity.

The electrical terminal 2 of primary coil 117 is grounded, whileelectrical terminal 1 can be connected to an AC generator. The twoelectrical terminals 3 and 4 of second primary coil 118 can be connectedto the inputs of a regulating circuit, which can change the value of theoutput voltage and/or its frequency in a known manner.

Secondary coil 114 is grounded on one side. The same also applies tosecond secondary coil 115. The two secondary coils 114 and 115 are wiredin parallel. Both coils are followed downstream first by rectifiercircuit 122 for rectifying the alternative current. Downstream fromrectifier circuit 122 is smoothing circuit 123, in the form of a knownLC filter. The output of smoothing circuit 123 is connected with coating8 with limited conductivity on roller 5.

During operation, secondary coil 114 moves relative to primary coil 117.Therefore the alternating voltage of primary coil 117 can induce asecondary voltage in secondary coil 114 through the air gap between thetwo magnetizable cores of receiving devices 113 and 116, said secondaryvoltage being supplied directly to coating 8 with limited conductivityafter being rectified by rectifier circuit 122 and smoothed by smoothingcircuit 123. The voltage induced in secondary coil 114 is tapped off bysecond secondary coil 115 and is induced in the opposite direction insecond primary coil 118, which can be connected by its electricalterminals 3 and 4 to a regulating circuit that can control the ACvoltage source in such fashion that the same DC voltage is alwaysapplied to coating 8 with limited conductivity on roller 5.

What is claimed is:
 1. A method of electrostatic charging of oppositesides of an at least one material web with charges of opposite polaritycomprising:moving the at least one material web through a nip between apair of parallel rotatable rollers having an outer semiconductive layerdefining the nip which contacts the opposite sides of the at least onematerial web; and charging the opposite sides of the at least onematerial web with opposite polarities by conduction from a voltagesource having opposite polarities to provide the opposite polaritiesrespectively, through the rollers to the outer semiconductive layers byrolling contact during moving of the at least one material web throughthe nip.
 2. A method according to claim 1, wherein:the rollers arepressed against the opposite sides of the at least one material web withan adjustable force.
 3. A device for electrostatic charging of oppositesides of at least one material web with charges of opposite polaritycomprising:a pair of rotatable parallel conductive rollers; asemiconductive outer layer disposed on each conductive roller; a nipdisposed between the outer layers of the rollers with the at least onematerial web moving through the nip with opposite sides of the at leastone material web being in contact respectively with the outer layer of adifferent one of the pair of rollers; and a voltage source in electricalcontact with the rollers which provides by conduction charges of theopposite polarity to the conductive rollers to charge the opposite sidesof the at least one material web with different polarity during rollingcontact through the nip of the outer layers with the opposite sides ofthe at least one material web.
 4. A device according to claim 3,wherein:the rollers have an axis disposed in a machine frame; and theaxis is connected electrically conductively to the conductive rollersand is electrically insulated from the machine frame.
 5. A deviceaccording to claim 4, wherein:an electrical voltage from the voltagesource is applied to a pressure bearing located at an end of the axis.6. A device according to claim 4, further comprising:a primary coilmounted concentrically with respect to the axis; and a secondary coilattached concentrically to the axis at one end thereof and turningtherewith, and the secondary coil is electrically connected to the axisand through a rectifier circuit to the semiconductive outer layer.
 7. Adevice according to claim 6, further comprising:a smoothing circuit forsmoothing a pulsating DC current produced by the secondary coil which iselectrically coupled to the rectifier circuit and the semiconductiveouter layer.
 8. A device according to claim 6, wherein:the primary coilcomprises two primary coils and the secondary coil comprises twosecondary parallel connected coils, one of the two secondary parallelconnected coils being rotatable and located opposite one of the twoprimary coils.
 9. A device according to claim 7, wherein:the primarycoil comprises two primary coils and the secondary coil comprises twoparallel connected secondary coils, one of the secondary coils beinglocated opposite one of the primary coils.
 10. A device according toclaim 3, wherein:spacing between the nip is adjustable.
 11. A deviceaccording to claim 4, wherein:spacing between the nip is adjustable. 12.A device according to claim 5, wherein:spacing between the nip isadjustable.
 13. A device according to claim 6, wherein:spacing betweenthe nip is adjustable.
 14. A device according to claim 7,wherein:spacing between the nip is adjustable.
 15. A device according toclaim 8, wherein:spacing between the nip is adjustable.
 16. A deviceaccording to claim 9, wherein:spacing between the nip is adjustable.